Don't return uninitialized memory from gvir_designer_domain_add_interface_full()
Don't return uninitialized memory from gvir_designer_domain_add_interface_full()

When an unknown NIC type is passed to
gvir_designer_domain_add_interface_full(), 'ret' would be returned
uninitialized to the caller.

libvirt/libvirt-designer,libvirt/libvirt-designer